The present invention relates to a waveguide junction composed of four rectangular waveguide branches, or arms, connected in the form of a cross, with a partially reflecting obstacle being disposed diagonally in the junction region to produce a directional coupler.
Waveguide connections employing multimode waveguides are known which employ directional couplers in which a suitably dimensioned dielectric slab is placed diagonally in a cruciform junction of the waveguides. This slab acts as a partially permeable mirror with respect to electromagnetic waves. A multimode directional coupler in rectangular waveguide technique has formerly been described by B. Wardrop: `A quasi-optical directional coupler` in The Marconi Review, Vol. 35, No. 185, 2.sup.nd quarter 1972, pp. 159-169. In order to obtain a 3 dB coupling the permittivity of the dielectric slab has to be chosen between 3.4 .ltorsim. .epsilon..sub.r .gtorsim. 4.0.
FIG. 1 is a schematic representation of such a directional coupler. The four waveguide branches, or arms, 1, 2, 3 and 4 are usually combined into a precisely rectangular cross junction. However it is also possible to have them intersect at any desired angle. As can be seen in the sectional view of FIG. 1, a dielectric slab S is disposed in one diagonal of the junction and is mounted in a simple manner in that its ends usually protrude from the lateral sides of the waveguide junction.
In the operation of this coupler, a wave impinging in waveguide branch 1 is divided, in the ideal case, at dielectric slab S and distributed to the two waveguide branches 3 and 4 without being reflected back into waveguide branch 1 or diffracted into waveguide branch 2.
This ideal case is approached more closely, the more quasi-optically the waves propagate in the waveguides, i.e. the farther the operating frequency f is removed from the cutoff frequency f.sub.c of the useful mode in the waveguides. Normally, however, waveguides are operated with the H.sub.10 mode at frequencies only slightly above the limit frequency f.sub.c ; the conventional rectangular waveguide bands are dimensioned, for example, for frequencies of 1.25 f.sub.c &lt; f&lt; 1.9 f.sub.c. For this monomode range, however, the known directional coupler shown in FIG. 1 is not well suited. This is so because the dielectric slab introduced in the diagonal is mismatched to a considerable degree at these frequencies, giving rise to substantial reflections back into waveguide branch 1 and diffractions into waveguide branch 2.